The invention relates broadly to fuel management systems of internal combustion engines. More particularly, the invention relates to an auxiliary circuit for limiting the top speed of an internal combustion engine. Still more particularly, the invention relates to the suppression of fuel delivery to fuel-injected internal combustion engines above a predetermined speed.
Known in the prior art are systems for limiting the top speed of an internal combustion engine, for example by interrupting or short-circuiting the ignition with a suitable switch which may be actuated by centrifugal force and which is normally a part of the distributor rotor. A system of this type involves mechanical tolerances and has the disadvantage that, prior to the actuation of the switch, a mechanical displacement is required. Accordingly, the known apparatus is able to limit the engine speed only within a certain finite rpm domain but cannot act at a precisely defined limiting speed. Furthermore, the interruption of ignition pulses, while preventing combustion of the flammable mixture in the engine, does not prevent the generation of a combustible mixture which continues to be delivered into the engine and passes into the exhaust system. When ignition resumes, explosions of this uncombusted mixture may take place in the engine and the exhaust system and may cause excessive heating of, for example, an after burner or catalyzer, causing its destruction. Furthermore, the combustion of unburned mixture in the exhaust system results in undesirable exhaust gas emissions.
Also known in the art are electronic engine speed limiters, especially associated with fuel injection systems. It is a disadvantage of all these electronic systems that they normally require substantial engagement of the fuel injection system, thereby making a retrofitting virtually impossible.
An electronic rpm-limiter encounters particular difficulties if it is associated with a fuel injection system in which the fuel injection control pulses are generated by a control multivibrator in proportion to air flow rate and engine speed. In such a system, the duration of the fuel injection control pulses is equal to the time constant of the multivibrator, which in turn is defined by the inductivity of a transmitter which is actuated by induction tube pressure and which is connected in the feedback branch of a monostable multivibrator. In a system of this type manufactured by the applicant's assignee and named "D-Jetronic", the engine speed only constitutes an auxiliary control variable which is sensed by two or more trigger contacts within the distributor which generate a pulse train in which the period between pulses is subject to relatively large fluctuations.